Why is it that we enjoy round numbers? I imagine there is something in our brains that prefers things to be orderly (even if our bodies never comply by picking up our dirty socks). Case in point: On Saturday I ran a full ten miles for the first time. For some reason that is much more significant to my brain than my previous best, which was 9.5 miles. And since a mile is just an arbitrary distance anyway, it's pretty irrational. But irrationality kind of seems to define us anyway.